High
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Efficiency in
Oil-engine Practice
Exclusive Details of the New Lanova Engine, Which Gives an Unusually High Mean Effective Pressure and has Performance Characteristics Closely Comparable with those of Petrol Engines. A Low Fuel Consumption
IN several modern compression-ignition engines the Acro system is embodied. This is, of course, a layout, including an air chamber ; the bulk of the compressed air is forced into the chamber through a funnel. As the air is leaving the chamber through the funnel it is meeting the injected fuel and the mixture. The oxygen is thus given the best opportunity for coming into contact with the fuel particles, so promoting good combustion.
The originator of this Acro system was Franz Lang, of Munich. His protracted research work has now resulted in the evolution of a new type of compressionignition engine, which, according to the details we have obtained, offers a higher standard of general performance than has yet been obtained in commercial practice. This power unit is of the solid-injection type, and has the advantage of being simple in layout.
A study of the vertical cross-section of this engine reveals the fact that the piston at the top of its stroke has only the required mechanical clearance from the cylinder head; in the last-named is embodied the main combustion space; therein is situated the horizontal injector and exactly opposite this an air-storage chamber of controllable volume. Into the latter air is driven during compression, and a certain amount of it is retained in the storage space.
It is important to note that the main combustion space in the cylinder head is in area somewhat smaller than the cylinder itself. The object of the employment of this space is to prevent residues of combustion and dirt from fouling the cylinder walls.
A study of the section of the cylinder head (in plan) discloses the heartshape of the combustion space with the horizontal injector at the "heart point" and opposite to it the storage chamber. As the piston rises
the air fills the combustion space and the storage clamher; towards the completion of compression a portion of the air in the storage chamber will commence to move outwards. As it impinges upon the "point of the heart" it is divided into two streams, rotating in the opposite direction to that of the fuel spray.
In this way the air stream passes the tip of the injector in such a manner that the atomized fuel particles are brought into contact with the maximum amount of oxygen, thereby ensuring complete combustion. This, in turn, gives rise to efficiency throughout the expansion stroke.
Starting is effected without the need for heater coils, and the air-storage chamber has its volume controlled by hand or by mechanical means, and is adjusted according to the load of the engine. It is noteworthy that the compression is comparatively Tow for an oil engine—namely, 370 lb. to 375 15. per sq. in.
One of the major difficulties encountered in the past has been the need for high air-excess figures in order to guarantee complete combustion. As the mean effective pressure is a function of the air-excess figure— hitherto somewhat greater than 1.35—the ine,f). has been rather lower than is usual in petrol-engine practice, where such a high air excess is not encountered.
On the other hand, if high compression ratios and high-pressure rises were utilized to compensate for the air-excess figure, the mean effective pressure could he increased, but the maximum pressure would rise unreasonably. In consequence, the weight of the engine would become greater, so that a compronlise has had to be struck.
Hitherto oil engines have not had so great a power output per unit of piston displacement as have their petrol. counterparts. In addition, the high ratio of maximum pressure to mean effective pressure has given rise to a certain roughness of running. •
To overcome these difficulties has been ' e object of Mr. Lang in his research work on the Lanova engine. His first step was to decrease the air-excess figure by improving the inter-mixture of the fuel and air. It was found in practice that the combustion-chamber layout, already outlined, resulted in the fuel particles being brought into contact with almost 100 per cent, of the available air, because, by the rotational motion, the comparatively cold air in each of the cores of the two whirls is led into the outer zone and then moves downward. In this way air-excess figures so low as 1.1 can he realized, with a result that higher m.e.p. figures are obtained.
High Mean Effective Pressure.
This means that the Lanova engine has a In.e.P. figure corresponding to that of a petrol engine with a carburetter setting suitable for optimum economy. This holds good in the case of the first experimental Lanova engine, but in production types it is claimed that the m.e.p. will be equivalent to that of a petrol engine with a setting suitable for maximum output.
With this high m.e.p. comparatively low maximum pressure can be combined, because of the low compression ratio—approximately 12 to 1 or 13 to 1—that is, approximately 20 per cent. to 30 per cent, lower than in the majority of other oil engines. Further, the pressure-rise ratio is lower than that of most other high-speed oil engines.
A combination of these circumstances means a certain sacrifice of theoretical therntal efficiency, but by reason of the improvement in combustion this loss is more than balanced. The maximum compression pressure is in the order of 380 lb. to 440 lb. per sq. in. and the pressure-rise ratio is about 1.5, so that the miximum pressure in the Lanova will be in the order of 540 lb. to 630 lb. per sq. in., with mean effective pressures of 115 lb. to 125 lb. per sq, in.
The high degree of turbulence obtained in this engine is generated without unduly increasing the pumping losses, because only a smaller portion of the air charge has to be forced through the restriction between the combustion chamber and the storage chamber. In this way the design is well suited to high-speed work.
Variable Cohtpression.
The variable-volume storage chamber enables compression to be controlled somewhat, and this naturally benefits the ease of starting. The injection pressure is in the order of 1.150 lb. to 1,400 lb. per sq. in., and the fuel is injected by a special pump and nozzle.
To demonstrate the efficiency of the turbulence a test piece of sheet metal, about .008 in. thick, was attached to a plug passing through the wall of the combustion space, so that the metal projected into the area of the swirl anti its section was in a vertical position. The force was sufficient to bend almost at a right angle the metal test piece.
During tests a single-cylindered engine, running at about 1,400 r.pan., gave a m.e.p. of 115.2 lb. per sq. in., a fuel consumption of .433 lb. per b.h.p.-hour, at an air-excess figure of 1.125. It is anticipated that the fuel-consumption figure will be in the order. of .39 lb. to .4 lb. per b.h.p.-hour, according to the load, when the engine is built as a multi-cylindered unit in its produc-. tion form, In spite of the unusually low air-excess figure combustion is practically free from smoke and soot, as proved by exhaust-gas analyses.
This combustion system has been incorporated in certain engines of other design, and, we are informed, has given marked improvements as regards m.e.p. and fuel consumption